Doncaster Mainline:
https://www.flickr.com/photos/tarquinius_superbus/7463179132
Even by the standards of uninspiring municipals with uninspiring liveries, South Yorkshire Transport had a fairly dull brown/cream livery, which was (later) livened up with some red. But in a post-deregulation era of colourful independents, they repainted the double deckers on the busy cross-city 81/84 corridor in a vivid red/yellow livery and called it "Mainline".
That seemed to work so well that other routes got the treatment over the next few years (converting one route/corridor at a time, rather than haphazardly applying it to any old vehicle - something that First/Stagecaoch repaints ought to pay more attention to), giving the impression of an upgrade (even when it was just the same vehicles as before) - the virtuous cycle continued until the point that the whole operation became "Mainline", and the yellow/red livery adopted across the main fleet.
However, Rotherham and Doncaster initially got their own versions of "Mainline" - Rotherham had a deep blue and yellow scheme in a similar pattern to the Sheffield red/yellow, but Doncaster went one further and adopted a main grey scheme with red and yellow used a lot more sparingly (see picture linked above).
IMHO it looked wonderful - the bright colours standing out more through their sparing use (I don't mind some of the vivid pink/turquoise/ orange etc that modern operators use but it works a lot better as a relief to a main duller colour) - red wheels made it stand out too.
There was no gimmick, no cartoon characters or complicated swoops or wacky pun, it was easy to sort out for replacement panels, I didn't see a bad example of a repaint (compared to some modern liveries where certain local depots can apply really haphazard interpretations of what they think Head Office asked for).
I think the Doncaster scheme looked quite futuristic and would have worked well on modern vehicles (where the large black areas taken up by modern glazing etc mean there's little space for a livery to really work - flashes of bight yellow and red would be a nice contrast upon an E400 etc).
Most enthusiasts I've spoken with seemed to
really dislike the Doncaster scheme and it didn't last very long - the whole fleet in South Yorkshire was "unified" into something based on the Sheffield livery but with narrow bands of deep blue and grey as a nod to the Rotherham/Doncaster liveries - when First took over in the late '90s any repaints became the simple red/yellow that Sheffield had started out with in the first place. Though the entire "Mainline" era was less than a decade, and went through the changes mentioned above, it's still the name that people use for "First South Yorkshire" - it seems to have a resonance in that I've even heard people use it who only moved to this neck of the woods post-1998.
So, I'll nominate the
Doncaster Mainline livery on the grounds that it is a "lost" livery (i.e. no longer around), it was the livery of the whole depot (rather than being restricted to one vehicle or only a temporary advertising livery) and lasted for a few years.
(As a "next best" post-deregulation livery, I'll suggest the "express" scheme that Kelvin Scottish had on the Cumbernauld corridor - a reverse of the regular "yellow front, blue rear" scheme - I thought that looked fabulous at the time)
